1. Field of the Invention
The present invention relates a photometric device incorporated in a camera, especially in a lens-fitted photo film unit, for measuring subject brightness, and a method of mounting a photo sensor on a circuit board, especially on a flash circuit board in a lens-fitted photo film unit, for measuring subject brightness.
2. Background Arts
A variety of lens-fitted photo film units have been widely known as a kind of single-use camera that contains an unexposed photo film cartridge and have simple mechanisms for taking photographs. Among the lens-fitted photo film units, hereinafter called simply the film units, those with a flash device have been widely used.
The flash device of the film unit is constituted of a printed circuit board and circuit elements mounted on the board. To facilitate assembling the flash device at a low cost and accommodating it compactly in the unit body, the circuit elements are integrally mounted on one side of the circuit board, and are connected to each other through printed circuit or printed wiring made of copper foil formed on the other side of the board. That is, terminals of the circuit elements are put from the mounting side through the circuit board, and are concurrently connected to the printed circuit by reflow-soldering or the like. Thereby, the circuit elements are secured to the circuit board at the same time. The circuit board of the flash device, hereinafter called the flash circuit board, is located between a body basic portion and a front cover of the film unit, with its mounting side oriented to the body basic portion, so that the circuit elements are accommodated in a recess between a film roll chamber and an exposure chamber of the body basic portion. Accordingly, the other side of the flash circuit board having the printed circuit thereon, hereinafter referred to as the print side, is oriented forward and opposed to an inside surface of the front cover.
Since the film unit is expected to be provided at a low price, the shutter speed and the aperture size of the film unit are fixed to simplify the photographic mechanism and thus reduce the production cost. Therefore, under- or over-exposure is likely to occur in the film unit. In order to produce photo-prints of acceptable exposure conditions from those over- or under exposed negatives, amount of printing light is controlled in a compensating way. However, it is impossible to deal with extremely over- or under-exposed negatives even by the exposure correction in printing.
To solve this problem, a simple automatic exposure control device the film unit has been studied in the art, that switches the shutter speed or the aperture size between two values based on a threshold value of subject brightness. Just changing the shutter speed or the aperture size between two values makes it possible to photograph most scenes with such exposure conditions that are correctable to make prints with acceptable exposure conditions by adjusting the printing light.
To measure the subject brightness, the automatic exposure control device needs a photometric device. In terms of space efficiency and wiring, it is preferable to form a circuit of the exposure control device on the flash circuit board and mount a photo sensor, such as a CdS cell, on the flash circuit board by soldering. However, as described above, the mounting side of the flash circuit board is oriented rearward to the body basic portion of the film unit, so the photo sensor cannot be mounted on the mounting side in the same way as other circuit elements of the flash device, because a photoreceptive face of the photo sensor would be directed inward in that case. It is possible to mount the photo sensor on the body basic portion or the front cover with its face directed outward, and connect it through lead wires or the like to the exposure control circuit on the flash circuit board. However, because this solution needs complicated wiring, production efficiency as well as space efficiency would be lowered, and thus the production cost would be raised.
On the other hand, a film unit with a taking lens of a long focal length has also been known as a telephoto type film unit. The telephoto type film unit is disclosed for example in U.S. Pat. No. 5,628,039. Because the telephoto type film unit is designed to focus on a distant range way beyond reach of the small flash device of the film unit, the telephoto type film unit does not usually have the flash device. Therefore, the lower limit of subject brightness for the telephoto type film unit to photograph with acceptable or correctable exposure conditions is higher than the lower limit for a film unit of a standard focal length, e.g. 32 mm, and with a flash device.
Accordingly, it has been considered to provide the telephoto type film unit with a low brightness warning function for warning the photographer when the subject brightness is below a level predetermined in accordance with the film speed, for the sake of avoiding extreme under-exposure. Also in this case, a photometric device must be mounted on the film unit.
In either case, it is necessary to measure the subject brightness in an area within a photographic field of the taking lens. Since the field angle of the long-focal length taking lens is remarkably narrower than that of the standard focal length lens, the photometric area and thus the acceptance angle of the photo sensor for the telephoto type should also be set narrower. For example, to define the photometric area of the photo sensor to be approximately ⅔ of the photographic field of the taking lens in a telephoto type film unit where the focal length is 120 mm and the contained photo film cartridge is of 135 type whose effective frame size is 22.5 mm in the short side, the acceptance angle on the photo sensor should be 7.125 degrees. This acceptance angle is about xc2xc compared to that suitable for a standard film unit where the focal length is 32 mm and the film is of 135 type.
To define the acceptance angle on the photoreceptive face of the photo sensor, a mask plate with a hole for limiting the incident light is placed in front of the photo sensor. The acceptance angle on the photo sensor may be defined by the diameter of the hole and the thickness of the mask plate, i.e. the depth of the hole. To obtain the above acceptance angle of 7.125 degrees, the depth-to-diameter ratio is 1:8.
Where the photoreceptive face of the photo sensor has a diameter of 6 mm, and the diameter of the hole of the mask plate is 6 mm, the depth of the hole should be 48 mm according to the above depth-to-diameter ratio. However, taking account of the mounting space for the photo sensor in the unit body, the depth of the hole should not be more than 8 mm. Where the depth of the hole is 8 mm, the diameter of the hole must be 1 mm, according to the above depth-to-diameter ratio. In that case, the amount of light falling on the photo sensor would be too small to measure the subject brightness reliably.
In view of the foregoing, an object of the present invention is to provide a method of mounting a photo sensor on a circuit board of a camera in a compact fashion at a low cost without the need for labor-consuming wiring, such that the photo sensor can measure subject brightness, wherein the circuit board has a print side with circuits printed thereon, and is mounted in the camera with the print side oriented toward an object side of a taking lens of the camera, and circuit elements are mounted on a mounting side of the circuit board that is opposite to the print side. The photo sensor has a photoreceptive surface on its front face, and a pair of terminals extending rearward from the photo sensor
Another object of the present invention is to provide a photometric device that is mounted in a film unit in a compact fashion at a low cost.
A further object of the present invention is to provide a photometric device wherein a sufficient amount of light enough for reliable photometry falls on a photo sensor of the photometric device at a very small acceptance angle.
According to an aspect of the invention, the photo sensor is mounted on the circuit board by putting the terminals through the circuit board from the print side to the mounting side; bending the terminals, after being protruded rearward from the mounting side, toward the circuit board; putting the bent terminals through the circuit board from the mounting side, to protrude tips of the terminals from the print side; and soldering the tips of the leading terminals on the printed circuits on the print side.
It is also possible to mount the photo sensor on the circuit board by forming an opening through the circuit board; putting the photo sensor on the mounting side of the circuit board, so as to expose the photoreceptive face through the opening; thereafter bending the terminals toward the circuit board; putting the bent terminals through the circuit board from the mounting side, to protrude tips of the terminals from the print side; and soldering the tips of the leading terminals on the printed circuits on the print side.
A photometric device of the invention comprises:
a photo sensor having a photoreceptive surface;
a photometric window formed in front of the photoreceptive surface; and
a mask member provided between the photometric window and the photoreceptive surface, with a constant thickness in an axial direction perpendicular to the photoreceptive surface, the mask member having a plurality of smaller holes than the photoreceptive surface, that are formed through from a front side that is exposed through the photometric window to a rear side that is opposed to the photoreceptive surface.
The acceptance angle on the photoreceptive surface is reduced by reducing the size of each hole where the thickness of the mask member in the axial direction is constant, while the total amount of light falling on the photoreceptive surface through the mask member is increased by increasing the number of holes of the mask member within an area corresponding to the photoreceptive surface.